XYLEM RESIN IN THE
RESISTANCE OF THE PINACEAE
TO BARK BEETLES
Richard H. Smith
PACIFIC SOUTHWEST Forest and Ranee
Experiment station Xylem resin-a supersaturation of resin acids in terpenesappears to play a paradoxical role in the relation of the Pinaceae
to tree-killing bark beetles. It has been suggested as the agent
responsible for the susceptibility of this coniferous family to
beetle attacks. And, at the same time, it has been linked with the
ability of the Pinaceae to resist bark beetles. The hosts of treekilling bark beetles are nearly all Pinaceae-Pinus (pine), Abies
(fir), Pseudotsuga (Douglas-fir), Picea (spruce), Tsuga (hemlock),
and Larix (larch). Significantly, among conifers, xylem resin is
most common and abundant in these Pinaceae. Bark beetles are
found on other families of conifers, but they are usually considered of minor importance.
Much experimental work has been done since the early reports
of an apparent association of resin with resistance. Nearly all
studies have dealt with attacking adult beetles; virtually no research has been directed at effects of resin on immature forms.
Most work has been with Pinus and Dep'roctonus, with lesser
attention to spruce, fir, Douglas-fir, and the other genera of bark
beetles.
This report summarizes the early findings, updates them with
results of more recent reports and, in some cases, reinterprets
these previous reports. This review deals only with bark beetles
that attack living trees and with xylem resin, although in a few
instances, it concerns resin-related chemicals as well.
This review follows the approach of Painter (1951), who proposed that plant resistance to insects depends on one or more
FOREST SERVICE
U. S.DEPARTMENT OF AGRICULTURE
P. 0. BOX 245. BERKELEY. CALIFORNIA 94701
Smith, Richard H.
1972. Xylem resin in the resistance of the Pinaceae to bark beetles.
Pac. Southwest Forest and Range Exp. Stn., Berkeley, Calif.,
7 p. (USDA Forest Sew. Gen. Tech. Rep. PSW-1)
Xylem resin of Pinaceae is closely linked with their resistance and susceptibility to tree-killing bark beetles. This review of the literature on
attacking adults suggests that all three resistance mechanisms proposed by
Painter-preference, antibiosis, and tolerance-are active in this relationship: preference by attraction, repellency, and synergism; antibiosis by
both chemical and physical properties; tolerance by healing and secondary
resinosis.
USDA FOREST SERVICE
GENERAL TECHNICAL
REPORT PSW- 1 I1972
Oxford: 453-145.7X19.92 Scolytidae: 174.7 Pinaceae: U547.914.
Retrieval Terms: Pinaceae; insect-host relationships; Scolytidae; resins; terpenes; attractants; antibiosis.
sides of a triangle-insect preference, antibiosis, and
tree tolerance-and attempts to show the action of
resin through these three mechanisms. Preference refers to the selection or avoidance of the plant by the
insect. Painter defined antibiosis as "the tendency to
prevent, injure, or destroy (insect) life." In this discussion, the physical and chemical manifestations of
resin will be considered separately. Tree tolerance,
with reference to Pinaceae and bark beetles, is an
active response of the tree to attack; and a brood of
beetles successfully develops without killing an exteriorly noticeable part of the tree.
Stark's review (1965) was oriented somewhat
along Painter's lines, but he omitted preference, and
he treated antibiosis from a different viewpoint by
using the classical entomological classification of
stomach, contaci, and fumigant toxicity. In discussing the nature of resistance, Callaham (1966) fob
lowed Painter's triangle. He proposed that the host
specificity of pine bark beetles is primarily attnbutable to interspecific resin variation, and he rests his
case largely on antibiosis.
The term allelochemics (Wittaker and Feeny 1971)
may be applied to much of the interaction of resin
and beetles described herein. It is another example of
the importance of secondary plant substances in
insect-host relationships,
Commercially, the resin acids are called rosin; the
terpenes, turpentine. Resin may also include small
amounts of related alcohols and aldehydes. The terpenes are volatile hydrocarbons and are usually considered the active portion of resin in relation to bark
beetle reaction.
Three main groups of terpenes are: (a) the monoterpenes, by far the most abundant in the xylem resin
of most Pinaceae, which are a 10-carbon molecule
and are quite volatile; (b) the sesquiterpenes, which
are a 15-carbon molecule and considerably less volatile than the monotelpenes; and (c) the diterpenes,
which are a 20-caibon molecule and even less volatile
than the sesquiterpenes. Each of the 18 different
monoterpenes has the same molecular weight and elemental composition, but different bonding or molecular configuration or both.
For a long time it has been known that both red
(D. valens Lee.) and black (D. terebrans [Oliv.] ) turpentine beetles are attracted to resin-even to fresh
paint thinned with turpentine. The attractiveness of
freshly cut logs to some bark beetles, notably the
spruce beetle (D.mfipennis Kifby) and Douglas-fir
beetle (D. pseudotsugae Hopk,), has been attributed
to resin volatiles. Johnson (1966) found lightningstruck ponderosa pines were readily attacked by the
western pine beetle (D. brevicomis LecJ. This phenomenon, which has been observed for other species
of Demlroctonus and for several species of Ips. could
be caused by reduced tree vigor. Since these attacks
are often made immediately after the lightning strike,
however, the tree's attractiveness could have been increased by the sudden release of the volatile portion
of resin or their chemical derivatives, at the time of
the damage to the tree.
Mirov' noted that in the laboratory the volatile
components of ponderosa pine (P. ponderosa Laws.
resin both attracted and repelled western pine beetle.
Gordon2 was the first to show differences in the response of this beetle to terpenelike compounds in
laboratory olfactometers. The difficulty of interpreting these older results may be attributed to the
use of gross and impure materials. Using an improved
and reliable field olfactometer and pure materials,
Bedard, et a!. (1969) found western pine beetle to be
generally unresponsive to the monoterpene components of ponderosa pine resin.
McMullen and Atkins (1962) found the Douglas-fir
beetle attracted to the odors of freshly cut Douglasfir (Pseudotsuga menziesif [Mirb.] Franco). No specific component was identified. However, since freshly
cut logs were much more attractive than older logs,
McMuflen and Atkins were apparently observing response to resin vapors. This observation was supported by Heikkenen and Hrutfioid (1965), who
found that a-pinene attracted Douglas-fir beetle and
fi-pinene repelled it. They theorized that the a- to
0-pinene ratio in Douglas-fir caused it to be attractive
or repellent. Working with the same beetle and tree
species, Rudinsky (1966a) reported a general ranking
of attractiveness as follows: camphene > gross resin >
a-pinene > limonene > four other terpenes or ter-
SIN ON BEETLE PREFERENCE
Studies to show that resin attracts or repels adult
beetles have not been conclusive. Even different experiments with the same beetle and tree species have
not yielded similar results. This discrepancy is not
unexpected, since behavior studies are complex and
difficult to interpret.
'
Mirov, N. T. A preliminary study of attraction with the
western pine beetle, Dendroetonus ftrerfcomi%Lee. 1928.
(UnpubL rep. on file Pacific Southwest Forest and Rang
Exp. Stn., Berkeley, Calif)
ord don, A. Tree infection in white fir and ecological studies
of insects attacking ponderom pine. 1932. {Unpubl. rep. on
file Pacific Southwest Forest and Range Exp. Stn., Berkeley,
Calif.)
penoid compounds, including fi-pinene. He concluded
(1966b) that "oleoresin appears to have a dual function for the Douglas-fir beetle: as an attractant orienting the beetles in flight and as a repellent. Resinosis is
the most important factor in the defense mechanism
of the tree."
Perttunen (1957) found that different concentrations of a-pinene changed the behavior of Hylastes
ater and Hylurgops palliutus. Chararas and Deschamps
(1962) speculated that resin and its volatile components of Scots pine (P. sylvestris L.), Norway spruce
(Picea abies L.), and Douglas-fir were associated with
host selection by Ips typographus L. and I. sexdentatus Boern. They based their case on earlier work by
Chararas on bark beetle response to terpenes and on
studies of the terpene composition of resin of the
three coniferous species. Kangas, et al. (1967) found
the gross extract of Scots pine phloem to be attractive to Blastophagus piniperda L.; they then identified the most active component as a-terpineol, an
alcohol derivative of a number of monoterpenes.
Data on the synergistic action of terpenes on
insect-produced pheromones continue to grow. In the
early period of bark beetle attraction work, the most
attractive condition was found to be feeding beetles
plus the tree. The most recent work narrows the
tree's contribution to individual monoterpenes from
resin. Bedard, et al. (1969) found that myrcene, a
naturally occurring terpene of pines, synergizes brevicomin, a pheromone produced by the western pine
beetle; myrcene by itself was not attractive. Other
studies (Pitman and Vit6 1969; Vit6 and Pitman
1968) imply somewhat similar synergistic relationships of host terpenes and the pheromones of the
mountain pine (D. ponderosae Hopk.), southern pine
(D. frontalis ZimmJ, and Douglas-fir beetles. Stark
(1968) touches upon the possible synergistic action
of host terpenes.
These advances in pheromones research and others
by Pitman (1966) and Wood, et al. (1968) offer a
new explanation about terpenes and preference. The
stereochemical theory of olfaction (Amoore 1963)
holds that the shape of a molecule rather than its
elemental composition is responsible for the reaction
of an animal to an odor. This could explain the attractiveness of natural terpenes and terpenoid compounds, since some of the terpenes are related to
pheromones or are similar to them in molecular structure (Silverstein, et al. 1967).
The evidence for attraction or repellency has not
been adequately resolved, but it is apparent that
resin, particularly the terpenes, is closely aligned with
host selection or avoidance. Possibly the host spe-
cificity of bark beetles is determined by the presence
in resin of one or more terpenes which do or do not
synergize the insect-produced pheromone. Likewise,
these findings suggest new pathways in breeding for
tree resistance by seeking LO eliminate a natural compound which is a strong synergist to insect-produced
attractants. As an example, ponderosa pines without
myrcene have been found.
ANTIBIOSIS BY RESIN TO ADULT BEETLES
Antibiosis, with reference to bark beetle attack,
has both physical and chemical aspects, and interaction between the two will be considered.
Physical Antibiotic Action
Some of the earliest observations on the adverse
effect of resin on bark beetles suggest attribution to
physical impediment; witness the term "pitch-out"
and "drown-out," used to describe unsuccessful attack. At least four measures of resin flow have been
devised: duration of flow, oleoresin exudation pressure (o.e.p.1, oleoresin exudation flow (o.e.f.), and
quantity of flow. Duration of flow is the time in days
during which measurable amounts of resin flow from
a standard wound; 0.e.p. is pounds of pressure per
square inch immediately after wounding; 0.e.f is
cubic millimeters per minute immediately after
wounding: quantity of flow is cubic centimeters of
resin exuding over the first 24-hour period per foot of
circumference from a full cross-sectional cut.
The quantity of flow as it affects beetle success
was considered by Mirov3 when he measured the
volume of resin flow of ponderosa pines in four areas
in northeastern California. He found generally that
stands with the highest average flow had less current
activity by western pine beetle than stands having the
lowest rate of flow.
Callaham4 found the duration of resin flow
shorter from high-risk ponderosa pines (those most
likely to be attacked and killed by bark beetles) than
from low-risk trees; the predominant beetle in his
study was the western pine beetle, with a much
smaller incidence of Jeffrey pine beetle (D. Jeffrey!
' ~ i r o v , N. T. A study of western yellow pine (Pinus ponderosa) as the host tree o f Dendroctonus brevicomis. 1928.
(Unpubl. rep. on file Pacific SW. Forest and Range Exp. Stn.,
Berkeley, Calif.)
allah ah am, Robert 2. Oleoresin production in the resistance
of ponderosa pine to bark beetles. 1955. (Unpubl. rep. on file
Pacific Southwest Forest and Range Exp. Stn., Berkeley,
Calif.)
Hopk.), on Jeffrey pine (P. jeffreyi Grev. & Balf.).
Callaham speculated that the quick subsidence of the
flow of resin allowed the beetle to proceed unhampered with feeding and ovipositing. On the other
hand, the prolonged flow from more resistant trees
either exhausted, repelled, or trapped the beetle and
prevented oviposition.
After one season of research on o.e.p., Vit6 and
Wood (1961) suggested that ponderosa pines having
high pressure survived bark beetle attack (primarily
mountain pine beetle, with some western pine beetle)
considerably more frequently than did those having
low pressure. A second season of beetle attack and
pressure measurements produced results of the same
kind but with less difference. The stand was secondgrowth ponderosa pine at about 6,500 feet elevation
in the central Sierra Nevada, the upper elevational
limit for ponderosa pine in this area. Stark (1965)
reported a similar study in which the western pine
beetle was predominant, and mountain pine bettle
occurred in much smaller numbers. This time little or
no differential in survival was noted between trees
with high and low pressure. Wood (1962), working
with the California five-spined engraver (I. confusus
Lec.) and ponderosa pine, found that trees with low
and high pressure survived attack equally well. However, he did find that successful attacks became established quickest in cut logs in which the pressure
dropped to zero or near zero.
Mason (1966,~ 1969) used the term "0.e.f." to
express the initial rate of resin flow on an amount/
time basis. His results suggest that the incidence of
successful attack by 1 avulsus Eichh. was greatest in
loblolly pine (P. taeda L.) with lower 0.e.f.; I. grandicollis Eichh., on the other hand, resembled I. confusus in being unable to tolerate any flow of resin. He
also obtained data which suggest fairly strong gene
control of 0.e.f. However, there was evidence that
stand and tree condition also influenced the initial
rate of resin flow.
Casual observations of several pine species have attributed unsuccessful attacks of both the red and
black turpentine beetle to quantity of resin flow.
Tests in which western pine beetle were forced to
attack ponderosa pines were the first to show experimentally the effect of resin quantity on beetle success
(Smith 1969). When the flow of resin for a 24-hour
period was changed in different portions of the same
tree, the success of beetle attack increased as resin
5 ~ a s o nRichard
,
R. Dynamics of Ips populations after summer thinning in a loblolly pine plantation: with special
reference to host tree resistance. 1966. (Unpubl. Ph.D. thesis
on file at Univ. Mich., Ann Arbor, Mich.)
flow decreased. The number of beetles also was an
important factor in determining the success of beetle
attack.
Viscosity and rate of crystallization of resin are
characteristics which could act as physical impediments, as Stark (1968) suggests, but experimental evidence has not been produced.
A recent study of smog-damaged ponderosa pine
in the southern California mountains sought to associate a number of resin characteristics with bark
beetle susceptibility (Stark, et al. 1968). Trees visually classified as heavily damaged by smog had, in general, lower resin pressure, lower rate of resin flow,
lower total volume of flow, and higher rate of crystallization. In another part of this study, the highest
percent of trees attacked by western pine beetle and
mountain pine beetle was found in the heavy smog
damage category. Putting the two parts of the studies
together, Stark, et al. (1968) concluded that smog
affects resin characteristics which then reduce the
trees' resistance to bark beetles. It should be pointed
out that the two sets of trees were not the same and
that the bark beetle attacks in the second set of trees
could have caused changes in the characteristics of
the foliage, which are used in classifying smog
damage.
Chemical Antibiotic Action
Mirov (1961) postulated that resin chemistry
could be the reason for the host specificity of western
pine beetle and Jeffrey pine beetle. Callaham (1966)
expanded and formalized Mirov's ideas and suggested
that host specificity depends on the beetle's ability to
tolerate the resin of hosts and inability to tolerate the
resin of nonhosts. He specifically indicated that differential toxicity is most likely in the turpentine constituents of resin.
Gordon6 was the first to show the terpenes and
terpenelike compounds to be differentially toxic to
adult western pine beetles. Smith (1963) found that
resin vapors of nonhost pines were significantly more
toxic than host resin vapors to the western pine,
mountain pine, and Jeffrey pine beetle. Resin vapors
of hybrids between host and nonhost were usually
intermediate in effect, and the differences were not
always significant; however, in laboratory feeding
tests, these hybrid resin vapors caused a significant
reduction in the feeding of adult beetles (Smith
1966a). Forced attacks under field conditions on
both altered and unaltered pines generally substanc o r d o n , A. op. cit.
TREE TOLERANCE TO BEETLE ATTACK
tiated the laboratory results. Hybrids between host
and nonhost pines were unsuitable hosts (Smith
1966b; Callaham and Miller7), and other tests showed
that western pine beetle could attack and oviposit in
cut logs of the same species and hybrids when there
was no flow of resin, thus again associating resin flow
with resistance.
A series of laboratory and field tests showed that
the individual monoterpene components of ponderosa pine were differentially toxic to the western
pine beetle (Smith 1965, 1966b, 1966c, 1969); this
pine species is the primary host of the beetle. Other
studies indicated a large variation among trees in a
stand and among geographic regions in the monoterpene composition of ponderosa pine xylem resin.
Only very slight differences in terpene composition
were found within individual trees over 30 years of
age (Smith 1964,1968).
The concept of tree tolerance is somewhat paradoxical, since bark beetles must kill all or part of a
tree in order for a successful brood to develop. However, tolerance may be understood as present in those
instances in which successful brood development
causes no external evidence of killing or causes an
active response by the tree, such as the production of
different tissue or chemicals. With bark beetles,
tolerance is a relative term, depending on the population density. Nevertheless, there are at least three
fairly well documented cases which might be classified as tolerance.
In an early study of the fir engraver ( S c o w s ventrails Lec.) on white fir (Abies concolor [Gord. &
Glend.] Lindl.), Struble (1957) noted that there is
often successful brood development without external
evidence of injury, although a patch of dead phloem
remained beneath the bark. Struble also noted that a
great many attacks are not successful in producing a
brood. Work by Berryman (1969, 1970) with this
insect on A. grandis (Dougl.) shows this tree response
to be a type of tolerance. Callus tissue develops to
heal the wound and a secondary resin system is
formed to produce a copious flow of resin which is
far greater than that normally encountered. Reid, et
al. (1967) found a somewhat similar condition with
lodgepole pine (P. contorta Dougl. var. latijiolia
Engelm.) in response to attack by the mountain pine
beetle. Callus tissue formation has been observed in
species of conifers with attacks of the red and black
turpentine beetle. But some uncertainty exists in
these investigations about the terms "resinosis,"
"secondary resin," and "traumatic resin."
Chemical-Physical Interaction
Smith (1966) attempted to treat the interaction of
chemical and physical impediment by forcing attacks
of western pine beetle on ponderosa pines with different monoterpene composition. The trees were altered to change the amount of resin which flowed
during different 24-hour periods. The effects of resin
quality and resin quantity could then be assessed by
regulating the number of attacking beetles. The results indicate that as resin flow decreases beetle success increases. But, at comparable resin flow, the degree of beetle success was markedly affected by the
quality of the resin as measured by its monoterpene
composition. The ranking of the monoterpene components for their effectiveness against beetle success
was limonene > 3-carene > (3-pinene = standard; a
standard tree is one without a high concentration of
any of these three terpenes. This ranking is similar to
that obtained from laboratory tests of the toxic effect of individual monoterpene vapors on adult
beetles (Smith 1965). Thus, the favorable effect of
quality may be negated by poor quantity, as evidenced by the previously cited observation that
beetles can be forced to attack and oviposit on nonresin-producing logs of nonhost species. Likewise,
poor quality may be offset by good quantity.
IMPROVING RESIN CHARACTERISTICS
Once a property of pine resin has been firmly associated with relative resistance, attempts to augment
it should be considered. Merker (1967) found that
irrigation and application of fertilizers increased the
resistance of pine to bark beetles. He attributed this
increased vigor to improved water balance. The better
water balance would, in time, directly increase the
production of resin. Conventional silvicultural practices, such as thinning to regulate growing stock, may
indirectly improve the resin production characteristics of trees which adversely affect bark beetle attack.
Lyon8 suggested the possibility of introducing a
chemical into the tree to increase the toxic or repel-
allah ah am, Robert Z., and John M. Miller. Studies o f the
resistance of pine hybrids to bark-beetle attacks, season o f
1951. 1952. (Unpubl. rep. on file Pacific Southwest Forest
and Range Exp. Stn., Berkeley, Calif.)
8l.yon, Robert L. Personal communication, May 1971.
I;
lent action of the natural components of resin. The
chemical could be introduced physically, or genetically, if a genetic basis could be found.
The level of the resin property might be increased
through tree selection or breeding. Selection could be
for both phenotypes and genotypes. Certain properties of resin-quality, synthesis, and 0.e.f-have been
shown to be under strong genetic control. It is not
difficult to envision the advantages of a breeding program, once the target properties have been determined.
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Wood, David L.
1962. Experiments o n the interrelationship between
oleoresin exudation pressure in Pinus ponderosa
and attack by Ips confusus (Lee.) (Coleoptera:
Scolytidae). Can. Entomol. 94(5): 473-477.
Wood, David L., Lloyd E. Browne, William D. Bedard, and
others.
1968. Response of Ips confusus t o synthetic sex
pheromones in nature.
Sci. 159(3821):
1373-1374.
The Author
RICHARD H. SMITH is in charge of this Station's research on the biology,
ecology, and control of destructive forest insects, with headquarters in
Berkeley. He holds B.S. (1942) and M.S. (1947) degrees from New York
State University, College of Forestry, and a Ph.D. degree (1961) from the
University of California, Berkeley.
EDITOR'S NOTE: General Technical Report is a new series of Forest
Service research publications that complements the three existing
series: Research Paper, Research Note, and Resource Bulletin. The
General Technical Report series serves as an outlet for information of a
technical nature but not necessarily the product of a specific piece of
original research or resource survey.
The Forest Service of the ,S. Department of Agriculture
. . Conducts forest and range research at more than 75 locations from Puerto Rico to
Alaska and Hawaii.
. . . Participates with all State forestry agencies in cooperative programs to protect and improve the Nation's 395 million acres of State, local, and private forest lands.
. . . Manages and protects the 187-million-acre National Forest System for sustained yield
of its many products and services.
.
The Pacific Southwest Forest and Range Experiment Station
represents the research branch of the Forest Service in California and Hawaii.